Subject : C4) Has there ever been an attempt or experiment to
reduce the strength of a hurricane ?

Contributed by Chris Landsea (NHC)

The U.S. Government once supported research into methods of
hurricane modification, known as
Project STORMFURY. For a couple
decades NOAA and its predecessor tried to weaken hurricanes by
dropping silver iodide - a substance that serves as a effective
ice nuclei - into the rainbands of the storms. During the
STORMFURY years scientists seeded clouds in Hurricanes Esther
(1961), Beulah (1963), Debbie (1969), and Ginger (1971). The
experiments took place over the open Atlantic far from land. The
STORMFURY seeding targeted convective clouds just outside the
hurricane's eyewall in an attempt to form a new ring of clouds
that, it was hoped, would compete with the natural circulation of
the storm and weaken it. The idea was that the silver iodide
would enhance the thunderstorms of a rainband by causing the
supercooled water to freeze, thus liberating the latent heat of
fusion and helping a rainband to grow at the expense of the
eyewall. With a weakened convergence to the eyewall, the strong
inner core winds would also weaken quite a bit. For cloud seeding
to be successful, the clouds must contain sufficient supercooled
water (water that has remained liquid at temperatures below the
freezing point, 0°C/32°F). Neat idea, but it, in the end,
had a fatal flaw. Observations made in the 1980s showed that most
hurricanes don't have enough supercooled water for STORMFURY
seeding to work - the buoyancy in hurricane convection is fairly
small and the updrafts correspondingly small compared to the type
one would observe in mid-latitude continental super or multicells.

In addition, it was found that unseeded hurricanes form natural
outer eyewalls just as the STORMFURY scientists expected seeded
ones to do. This phenomenon makes it almost impossible to
separate the effect (if any) of seeding from natural changes. The
few times that they did seed and saw a reduction in intensity was
undoubtedly due to what is now called "concentric
eyewall cycles". Thus nature accomplishes what NOAA had hoped
to do artificially. No wonder that the first few experiments were
thought to be successes. Because the results of seeding
experiments were so inconclusive, STORMFURY was discontinued. A
special committee of the National Academy of Sciences concluded
that a more complete understanding of the physical processes
taking place in hurricanes was needed before any additional
modification experiments. The primary focus of NOAA's Hurricane
Research Division today is better physical understanding of
hurricanes and improvement of forecasts. To learn about the
STORMFURY project as it was called, read
Willoughby et al. (1985).